The detection of chemical particles or biological particles (sequence of nucleotides, proteins, cells, etc.) requires the sensor to have high recognition specificity and optimum sensitivity.
Existing detection means are QCMs (quartz crystal microbalances) with a resolution of 1 ng/cm2 (nanogram per square centimeter), SPR (surface plasmon resonance) with a resolution of 0.05 ng/cm2 or fluorescence.
Other devices have been developed. Some of these use gravimetric analysis, i.e. they detect the shift in resonance frequency when a particle is deposited on the sensor, while others use field-effect detection, i.e. they detect the modification in the semiconductor properties of a CMOS-type sensor when an electrically charged particle is deposited on the sensor (V. Agache et al., “1.1 GHz silicon blade nano-electromechanical resonator featuring 20 nm gap lateral transducers”).
Among those using gravimetric analysis, mention may be made of the mass sensor developed by Professor Michael Roukes' team. This sensor is a doubly anchored beam made of silicon carbide (SiC), said beam being excited in an out-of-plane bending mode by magnetostatic transduction with magnetic fields of a few tesla.
This sensor has a resolution of around 7 zg (corresponding to the weight of a 4 kDa molecule, i.e. slightly less than 7 pairs of nucleotides), and has a mass sensitivity of around 0.96 Hz/zg. However, these results were obtained for weighing 30 xenon atoms deposited under a high vacuum (10−10 torr) at T=4.2 K on the surface of the oscillators. These experimental conditions make it difficult to use this system for biological purposes, in which field it is necessary instead to actuate the system directly in the medium containing the analytes to be detected.
At the present time, only a few electromechanical sensors employing gravimetric detection operate in a liquid medium.
These sensors have an out-of-plane vibration mode that entails the displacement of a large volume of liquid, thereby degrading the quality factor of the sensors. Furthermore, most often they use a detection means external to the detection device, which proves to be problematic in relation to the noise generated, and to reducing the overall size of the detection device.
Most other proposed solutions do not enable the interaction kinetics to be monitored in real time, i.e. in the medium in which the particles to be detected are located.
This is because they rely instead on determining the added mass on the basis of the frequency measurement, before and after deposition of the particles to be detected (an approach often called the “dip and dry” approach). In this case, labels are often used so as to correlate the measured frequency shift with the amount of mass grafted, by simply counting the nanoparticles on the surface of the resonator (by SEM or AFM).
Another approach consists in incorporating the liquid of interest into the actual sensor. Using this principle, an MIT research team (Scott Manalis et al.) has fabricated a lever provided with an internal fluid stream in which the liquid comprising the particles to be detected circulates. The lever is set into vibration by electrostatic coupling at 220 kHz in a vacuum, while the liquid flows, at a constant flow rate, in the fluid stream.
The drawback of this system lies in the method of detection employed (optical detection using a laser and a photodiode having 4 external quadrants), which contribute to the overall size of the system, and in the complexity of the process for fabricating these devices.
In conclusion, none of the approaches proposed hitherto, using gravimetric detection based on an MEMS/NEMS oscillator, is capable of producing a small portable detection device (i.e. comprising measuring means integrated with the sensor on the same chip) for real-time detection in a liquid medium.
However, there are devices for detecting particles by a field effect. Their use in a liquid medium has been known since the beginning of the '70s.
Many technological developments have enabled nanoscale CMOS sensors, such as silicon nanowires or carbon nanotubes, to be produced.
Based on the same principle of conductance modification by charges, these devices have demonstrated high sensitivity in biological interaction detection applications. The drawback of this kind of device is the fact that the sensor and the molecule to be detected are of comparable size (ranging from a few nanometers to a few tens of nanometers). The addition of one or more molecules on the detector therefore greatly disturbs it. The specificity is provided by the probe molecule grafted onto the surface of the device. This therefore requires expensive high-resolution lithography tools for defining nanoscale sensors.
Thanks to CVD (chemical vapour deposition) growth, it is now possible to detect biological molecule concentrations comparable to those that can be detected by conventional fluorescence methods: a few femtomoles of DNA; a few picograms per milliliter of proteins; or a single viral particle.
These results have also been reproduced on silicon nanowires fabricated by post-photolithography silicon etching processes. Although the dimensions of the wires obtained by these technologies (50 nm to several hundred nm) are not as small as those obtained by CVD growth (10-30 nm), the experimental results of biological interaction measurement are nevertheless very good: detection of 25 femtomoles of DNA has recently been demonstrated.
Very good results have also been achieved on carbon nanotubes or sheets or carbon nanotubes. This technology has the advantage of not requiring expensive technology, but benefitting from the nanoscale properties of objects in solution. For example, DNA concentrations of the order of picomoles have thus been detected.
However, only electrically charged particles may be detected by sensors based on electrical detection by the field effect, so that these sensors are not multi-purpose sensors.
The object of the present invention is to provide an inexpensive, multi-purpose portable detection device, i.e. one with a small size, allowing, using a single nanowire, the in situ detection, directly and with no labels, of the mass and the charge of the biological or chemical particles in the fluid that are fixed onto the nanowire, allowing rapid analysis and having a specificity and a sensitivity that are at least equivalent to the existing devices.